1. Field of the Invention
The present invention relates to a gas preconcentrator, and more particularly to a micro gas preconcentrator manufactured by MEMS technology.
2. Description of Related Arts
It is needed for a gas sensor with high sensitivity, high selectivity, fast response, low power and portability to detect the trace level of industrial pollution, high explosives, drugs, and chemical and biological toxic agents for field analysis. However, it is difficult to meet the above mentioned requirements relying solely on the gas sensor itself. Therefore, it is compulsorily for a micro-preconcentrator to be introduced in the front end of the gas sensor for improving the sensitivity, a micro-chromatography to be incorporated for improving the selectivity. As a result, a plurality of components is combined into an integrated gas detector.
The preconcentrator has been widely used for many years in analytical chemistry. It collects and accumulates one or more chemical species of interest by passing the low concentration vapor stream through the sorptive material for a period of time. The sorptive layer is subsequently heated and the thermally released analyte provides narrow desorption peaks with relatively high concentration. The preconcentrator can improve not only the sensitivity of the detector for 1-3 orders of magnitude, but also the selectivity of the detector to the special target by the chemoselective sorptive film.
Besides the intrinsic characteristics of the sorptive film, the concentration factor of the preconcentrator is predominately determined by flow rate, heating rate and the area of the sorptive film. For a given amount of gas molecules collected on the sorptive film, the faster the heating rate, the higher the gas desorption peak intensity and the narrower the full width at half maximum (FWHM). To realize rapid heating, the preconcentrator with low heat capacity is preferred, and in principle fabricated by MEMS technology. The literature of “Trends in Analytical Chemistry, 2008, 27(4):327-343” systematically summarizes the research status of the MEMS gas preconcentrator in recent years.
In all micro-preconcentrators, the two-dimensional diaphragm preconcentrator disclosed by U.S. Pat. No. 6,171,378 (as shown in FIG. 1A) has the smallest heat capacity. For the SiN diaphragm with the thickness of 0.5 μm, it can be heated up to 200° C. under the power of 100 mW for 10 ms and the FWHM of the gas desorption peak is only 200 ms. However, the above mentioned planar preconcentrator has prominent drawbacks. Firstly, the sorptive film has small area. Secondly, the flow rate is too low while preconcentrating. As a result, the preconcentration factor of the 2D planar preconcentrator is far lower than that of the conventionally tubular preconcentrator (which can refer to FIG. 4b in the literature of IEEE Sensor Journal, 2006, 6(3): 784-795).
U.S. Pat. No. 7,118,712 and U.S. Pat. No. 7,306,649 disclosed 3D preconcentrators. A plurality of perforations acting as gas flow channels are formed on a three-dimensional material with a substantial thickness, and the sorptive film is coated on the inner walls of the gas flow channels. Compared with the 2D diaphragm preconcentrator, the surface area of the sorptive film of the 3D preconcentrator can be increased by tens of times, and the gas flow rate thereof can also be enhanced greatly. The clamshell preconcentrator (as shown in FIG. 1B) is a typical representative of the 3D preconcentrators. It uses the fin-shaped parallel grooves as the sorption support structure for increasing the area of the sorptive film, resulting in equal preconcentration factor with respect to the tubular preconcentrator. The clamshell preconcentrator also has suspended membranes. The thin-film heater and the fin-shaped sorption support structure are respectively disposed on two sides of each membrane. In spite of the above mentioned heat insulation design, the heat capacity of the clamshell preconcentrator increases remarkably due to the fin-shaped bulk structure. Consequently, the FWHM of the gas desorption peak is extended to 2.3 s (which can refer to FIG. 4b of the literature IEEE Sensor Journal, 2006, 6(3):784-795).
In an integrated gas detector, a micro-chromatography is needed to be incorporated at the downstream end of a micro-preconcentrator. In such a case, the preconcentrator must also act as an injector of a conventional chromatograph. The preconcentrator is required to desorb the accumulated chemical molecules as quickly as possible. Otherwise, the GC peaks will be broadened, thereby reducing the performance of the chromatography. Obviously, the 3D preconcentrators elevate their preconcentration factor at the expense of rapid heating capability, thus can not meet the demands of an integrated gas detector.
The diaphragm of the 2D preconcentrator tends to break during rapid heating since the inlet/outlet holes provided on the glass lid is too small. The perforated structure of the 3D preconcentrators greatly increases the sectional area of the flow channel, thereby improving the flow rate. However, in the various embodiments disclosed by U.S. Pat. No. 7,118,712 (as shown in FIG. 1B), the three-dimensional sorption support structures are suspended on a layer of thin diaphragm. Hence, the diaphragm is subject to a large static stress and at the risk of rupture.